The present invention relates to a single- or multilayer product containing xcex1-olefin-vinyl alcohol copolymers and at least one xcex1-olefin-vinyl acetate copolymer, wherein the amount of vinyl acetate repeat units present in the xcex1-olefin vinyl acetate copolymer is in the range from 35 to 99.99% by weight. These repeat units have good flexibility which permits the processing of films or other thin-walled items which are transparent after stretching or thermoforming, and to a process for producing the same, and to their use as packaging films.
Shrink packaging, xe2x80x9cskin-packxe2x80x9d system processes and stretching processes are used in particular for packaging films. EVOH is a very brittle material and the corresponding films have poor flexibility. On the other hand, EVOH films have excellent gas barrier properties, making EVOH the preferred material for boundary layers in multilayer films. EVOH has also been proposed for fuel hoses.
WO-99/28124-A1 shows that stress-strain curves are highly dependent on the build of the film. High strain values may be achieved with multilayer builds of up to 61 layers, and these are compared with three layers. However, the technical requirements for producing a film of this type are high, and therefore difficult to achieve with EVOH-containing films.
Examples of modifications to the EVOH resins using nylon-type polymers are described in Ahn, Tae Oan; Kim, Chang Kee; Kim, Byung Kyu; Jeong, Han M O; Huh, Jung. Dep. Chem. Technol., Seoul National. Univ, Seoul, S. Korea. Polym. English. Sci. (1990), 30(6), 341-9. In Ahn, So-Bong; Jeong, Han M o, Department of Chemistry, University of Ulsan, Ulsan, S. Korea, Polymer (Korea) (1999), 23(6), 837-843 the miscibility of PVPST (vinylpyridine-styrene copolymers) with EVOH is described. However, the effect on flexibility was not studied. Miscible EVOH/polyvinylpyrrolidone mixtures are disclosed in Ahn, So-Bong, Jeong, Han MO. Department of Chemistry, University of Ulsan, Ulsan, S. Korea, Korea Polym. J. (1998), 6(5), 389-395.
EP-A1-0 309 095 describes blends of EVOH with certain amorphous polyamides, but does not report any increase in the flexibility of these blends compared with pure EVOH.
EP-A2-0 820 381 describes mixtures of EVOH with polar copolymers, such as ethylene-acrylate copolymer and ethylene copolymer, these having been modified with maleic anhydride.
EP-A1-0 440 535 describes mixtures of EVOH with a PE of density from 0.900 to 0.940 and with the reaction product of a carboxylic-acid-grafted polyolefin with an oligomeric polyamide.
EP-A1-1 022 309 describes mixtures of EVOH, ethylene-acrylate copolymer and ethylene-acrylate-anhydride copolymer, which was reacted with polyamides for the purpose of coinjection with PET to manufacture parisons for bioriented bottles.
WO-98/45367-A1 claims mixtures of EVOH with ethylene-vinyl acetate copolymers for producing items with increased environmental stress crack resistance, the ethylene-vinyl acetate copolymers used having VA contents of up to about 30%. No particular requirements are placed here on oxygen transmission or permeability.
U.S. Pat. No. 5,993,977 describes multilayer composites which, alongside a PE layer impermeable to water and a paper layer, also have a layer which is impermeable to gas and which is composed of a blend of EVOH with a polar ethylene polymer. This polar ethylene polymer is regarded as a modified PE. However, the blend component claimed in the present invention is a vinyl acetate polymer, which has been modified in a nonpolar manner (with ethylene).
EP-A 0 333 443 discloses multilayer products whose layer structure comprises three different ethylene-vinyl acetate copolymer layers (EVM layers), a barrier layer, an adhesive layer and a copolyamide layer.
DE-A 26 44 209 discloses a composite film made from a polypropylene (PP) base film and from an intermediate layer of modified polyethylene (PE) and finally from an outer layer of ethylene-vinyl alcohol copolymer (EVOH). It circumvents the known difficulties with stretching of a EVOH layer by stretching the EVOH layer and the intermediate layer only in one direction, whereas the stretching of the PP film is biaxial. This requires the production of a cast PP film with subsequent monoaxial stretching, its lamination to EVOH and an intermediate layer, and then the monoaxial stretching of this composite. A disadvantage of this process is that it is composed of more than one step. This process cannot give biaxial stretching.
U.S. Pat. No. 4,400,428 discloses composite films, which comprise biaxial polyamide films and also ethylene-vinyl alcohol copolymer layers and sealable layers.
EP-A 132 565 discloses oriented multilayer films which are produced by coextruding polyamide films and ethylene-vinyl alcohol copolymers and then are monoaxially stretched.
JP-A 2001 277 419 describes films which can be used for the packaging industry and which comprise biaxially oriented nylon-6 films and a composition composed of polyurethanes, a two-layer polyurethane component, and an LLDPE film. They have good resilience and low permeability.
JP-A 2001 277 417 discloses biaxially oriented nylon-6 films adhesive-bonded to unstretched polypropylene and having low oxygen transmission.
JP-A 2001 294 279 discloses other, very complicated systems where a biaxially stretched nylon layer is provided with a polyurethane anchor layer, with polyvinyl alcohol and with protective layers.
JP-A 09039172 teaches stretching EVOH biaxially and then subjecting it to a corona surface treatment, laminating PE to both of its sides, and providing it with a biaxially stretched polyamide layer.
JP-A 09 039 182 discloses the coextrusion of polyamide (PA) with PE for biaxially stretched films and lamination to EVOH, which also comprises some polyamide, no anchor layers are needed. However, the barrier layer also has to be corona-treated.
WO 9 703 822 discloses films of high strength and low moisture transmission, composed of a monoaxially stretched layer of polyamide and EVOH and of a biaxially oriented LLDPE layer. These two layers have to be bonded by means of an intermediate polyurethane layer.
JP-A 08 207 927 discloses composite films with paper-like character without aluminum foil, a thermoplastic layer being bonded to a biaxially oriented polyamide, or to PET, or to a polypropylene layer. Vapor deposition of SiO2 then takes place. The remaining layer structure comprises a EVOH or PVOH layer, which is finally bonded to two or more paper layers.
JP-A 01 255 535 laminates a biaxially oriented polyamide film to a coextruded film made from ethylene-vinyl alcohol copolymer and an ionomer (adhesive layer), to obtain a shrink film.
JP-A 06255054 discloses 3-layer films whose first layer comprises nylon-6, while the second comprises a blend of semiaromatic polyamide of MXD nylon 6007 type with ethylene-vinyl acetate copolymer and the third comprises a mixture of the materials of the first two layers. A small amount of maleic-anhydride-grafted ethylene-propylene copolymer is also added to the polyamide layers here in order to improve resistance to fracture on creasing.
Biaxially oriented polyamide blends (nylon-6,6 and MXD,6) are also disclosed in EP-A 0 540 293 as having better splittability.
JP-A 04 169 231 discloses a 3-layer film structure with a copolyamide layer, with an ethylene-vinyl alcohol copolymer layer and with another copolyamide layer. This film can be stretched biaxially, but only in a ratio of 3:1. When a polyamide of nylon-6 type is used the film becomes wrinkled after stretching. The implication is that combinations of high-melting polyamide and EVOH cannot be stretched biaxially without phase separation.
JP-A 04 131 237 discloses films made from ethylene-vinyl alcohol copolymer and nylon-6/12 blends including LiCl, these being laminated to biaxially oriented nylon-6 and to unstretched polypropylene in order to obtain films impermeable to water vapor.
JP-A 01 221 241 teaches stretching a PET film biaxially and then laminates it to a composite film made from EVA/EVOH/nylon/EVA, the composite films then being subjected to corona treatment. According to JP-A 01 221 241, EVA means an ethylene-vinyl acetate copolymer whose vinyl acetate content is not more than 35% by weight.
JP-A 62 083 144 discloses a packaging film made from a biaxially stretched ethylene-vinyl alcohol copolymer film which, after thermal treatment, is laminated to a biaxially stretched polypropylene film. A polyurethane layer is also required here as adhesive layer.
JP-A 09 220 761 discloses stretched PA/EVOH/PA films. A specific heat-conditioning program has to be complied with here prior to the stretching step.
JP-A 1998 003 5669 discloses PA and EVOH as starting materials for film extrusion processes. However, the product is merely double layers, which then can be stretched only after treatment in water.
In Polym. Networks Blends, 7 (4), 139-46 (1997), Nir et al. describe blends of ethylene-vinyl alcohol copolymer with an ethylene-vinyl acetate copolymer whose vinyl acetate content is 9% by weight.
The prior art discussed shows that a particular pretreatment is essential to permit stretching or thermoforming of a multilayer product, which contains a thermoplastic elastomer, such as polyamide or polypropylene, and ethylene-vinyl alcohol copolymer (EVOH). Although the biaxial stretching of films which comprise only polyamide is a well known process, the stretching of EVOH is, on the other hand, difficult, since it is very brittle and, due to its rapid crystallization, has only very restricted processing latitude. Therefore there is insufficient provision for the manufacture of thin, stretched film structures, unless they are manufactured by a multistage process composed of many steps. The many steps lead to disadvantages in cost-effectiveness.
An object of the present invention was to find blend components for EVOH, permitting stretching of the mixture which contain a blend component and EVOH. Another object of the present invention was to retain the underlying mechanical properties of the EVOH and its good barrier property.
A good barrier property primarily means that the product does not exhibit good oxygen transmission. However, a good barrier property also means the ability to maximize the prevention of access of other molecules, such as odorous constituents and chemicals, from the wares to be protected.
At the same time, the adhesion properties of the EVOH films should generally be improved in order to gain options for the manufacture of relatively simple multilayer films. Another object of the present invention is therefore to provide a multilayer product which, without further pretreatment, is transparent after the stretching process and also has low oxygen transmission.
Accordingly, the present invention is directed to a single- or multilayer product containing one or more xcex1-olefin-vinyl alcohol copolymers and at least one xcex1-olefin-vinyl acetate copolymer, wherein the amount of vinyl acetate repeat units present in the xcex1-olefin vinyl acetate copolymer is in the range from 35 to 99.99% by weight.